1. Field of the Invention
The embodiments discussed herein are related to a method of manufacturing a semiconductor device.
2. Description of the Related Art
Conventionally, in a semiconductor device (silicon carbide semiconductor apparatus) using a silicon carbide (SiC) semiconductor, an ohmic contact (electrical contact portion) of a silicon carbide semiconductor portion and a transition metal layer (electrode) is formed by heat treatment (annealing). A high temperature of about 1000 degrees C. or more is used to form the ohmic contact. Heat treatment methods include, for instance, furnace annealing, laser annealing, and lamp annealing. Heat treatment of a vertical device having an electrode on both surfaces of a semiconductor substrate will be described with reference to FIGS. 12 to 15 as an example.
In a vertical device, a contact for the respective electrodes has to be formed in both surfaces of the semiconductor substrate. Therefore, as depicted in FIGS. 12 to 15, after a device structure 102 of the semiconductor front surface side is formed, heat treatment is performed after a back surface contact electrode 103 of the substrate back surface side has been formed. FIG. 12 is a cross-sectional view schematically depicting the state of a conventional vertical device during furnace annealing. With furnace annealing 104, although the entire back surface contact electrode 103 can be heated evenly, a semiconductor substrate (semiconductor wafer) 101 is also entirely heated uniformly. As a result, a problem arises in that device properties and fabrication materials degrade, putting constraints on the sequence of manufacturing processes. A hatched region indicated by reference numeral 103b represents the back surface contact electrode (silicide layer) after annealing (similarly for FIGS. 13 to 15).
FIGS. 13, 14, and 15 are cross-sectional views schematically depicting the state of a conventional vertical device during laser annealing. In the laser annealing, as depicted in FIG. 13, after the device structure 102 of the front surface side is formed, for example, the thickness of the semiconductor wafer is thinned to reduce conduction loss and in a back surface of the thinned semiconductor wafer 101, the back surface contact electrode 103 is formed. While laser 105 having a reduced spot diameter sweeps (indicated by arrows), predetermined regions of the back surface contact electrode 103a before annealing are sequentially irradiated to form a silicide. As a result, the back surface contact electrode 103 alone can be uniformly heated, even in cases where the thickness of the semiconductor wafer 101 is thinner at some portions, device surfaces are uneven, or slopes are present, as depicted in FIGS. 14 and 15, for example.
FIG. 14, for example, depicts a state where, by a trench 106 that penetrates a semiconductor wafer 101 in a direction of depth and is formed along a dicing line (not depicted) between regions that become a semiconductor chip 111, a side surface of a region that becomes the semiconductor chip 111 is slanted at a predetermined angle with respect to the wafer surface to form a tapered shape. FIG. 15 depicts a state where the strength of a thinned semiconductor wafer 101 is enhanced by, for example, reducing the thickness of only a predetermined region 101a by a trench 106 of a predetermined depth and leaving other portions 101b to have a significant thickness to prevent cracking caused by handling during wafer delivery, internal stress of the semiconductor wafer 101, etc.
To form an ohmic contact in a semiconductor substrate formed of silicon carbide (hereinafter, silicon carbide substrate), a method of vapor depositing a transition metal layer in a contact on the silicon carbide substrate and heating the entire silicon carbide substrate by rapid heat treatment at 1000 degrees C. for 2 minutes to form a silicide contact electrode with a high carbon content has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 2009-177102 (paragraph 0017)).
Another method has been proposed where after a nickel (Ni) layer is formed on a silicon wafer, hydrogen (H2) gas is introduced into the chamber creating a hydrogen gas atmosphere in the chamber, and a susceptor is heated to 450 to 550 degrees C. by a heater to heat treat the silicon wafer (for example, refer to Japanese Laid-Open Patent Publication No. 2011-066060 (paragraphs 0037 to 0040)). In Japanese Laid-Open Patent Publication No. 2011-066060, impurities in the nickel film are removed by hydrogen atoms entering the nickel layer, facilitating the reaction between silicon atoms in the wafer and nickel atoms in the nickel layer.
A further method has been proposed where after a titanium (Ti) layer, an aluminum (Al) layer, and a silicon layer are sequentially formed on a silicon carbide substrate by sputtering to form a contact electrode, annealing via laser is performed whereby the titanium, aluminum, and silicon included in the contact electrode and the silicon and carbon included in the silicon carbide substrate form an alloy (for example, refer to Japanese Laid-Open Patent Publication No. 2012-099599 (paragraphs 0042 to 0044)).
Yet another method has been proposed where an oxide film (SiO2), quantum dots formed from silicon, and a nickel (Ni) thin film are sequentially stacked on a silicon substrate and subject to remote hydrogen plasma processing for 5 minutes at a frequency of 60 MHz and a very high frequency (VHF) electrical power of 200 W to 500 W to form nickel silicide dots from a stacked film formed from the quantum dots and nickel thin film (for example, refer to Republished Japanese-Translation of PCT Application, Publication No. 2009-118783 (paragraphs 0056 to 0061)).
Nonetheless, with Japanese Laid-Open Patent Publication Nos. 2009-177102 and 2011-066060, portions forming the ohmic contact (i.e., the transition metal layer, or the interface of the substrate and the transition metal layer) cannot be heated selectively, the entire substrate (the entire device) is uniformly heated. Therefore, as described above, interface properties of the semiconductor portion and gate insulating film, and fabrication materials of the device may degrade. Further, constituent portions formed of a material having a lower upper temperature limit than the heat treating temperature cannot be formed before the heat treatment for forming the ohmic contact. For example, constituent portions formed of resin or having a low melting point metal such as aluminum have to be formed after the heat treatment for forming the ohmic contact.
In Japanese Laid-Open Patent Publication No. 2012-099599, by reducing the spot diameter of the laser 105, a predetermined region can be selectively heated (refer to FIGS. 13 to 15) and therefore, the problems associated with Japanese Laid-Open Patent Publication Nos. 2009-177102 and 2011-066060 can be resolved. Nonetheless, the efficiency of heating decreases if the distance from the converging lens (not depicted) that converges the light of the laser 105, to the irradiation position of the laser 105 deviates. Therefore, as depicted in FIGS. 14 and 15, in cases where the laser 105 is irradiated on a trench side wall that is slanted with respect to the wafer surface to form a tapered shape, or is irradiated on a steep chip side wall substantially orthogonal to the wafer surface, the distance from the converging lens to the irradiation position is not constant across the entire surface of the back surface contact electrode 103a consequent to the unevenness of the device surface. As a result, throughput may decrease since irradiation of the laser 105 has to be performed according to conditions corresponding to each irradiation position.
Further, in Japanese Laid-Open Patent Publication No. 2012-099599, irradiation voids occur consequent to deviation of the laser irradiation position, whereby contact resistivity becomes inconsistent; and near the transition metal layer, constituent portions (e.g., gate insulating film, etc.) other than the transition metal layer are heated, whereby device properties may degrade. Furthermore, if the surface area of the transition metal layer is smaller than the area corresponding to the spot diameter of the laser, a problem arises in that the transition metal layer alone cannot be selectively heated.
Moreover, for example, when a trench is formed that penetrates the semiconductor wafer in a direction of depth, a stage on which the semiconductor wafer is mounted, a support substrate supporting the semiconductor wafer and adhesive, etc. are exposed at the trench bottom. In this state, if furnace annealing or laser annealing is performed as in Japanese Laid-Open Patent Publication Nos. 2009-177102, 2011-066060, and 2012-099599, members exposed at the trench bottom degrade, outgassing, particulate formation, etc. occur, and defects such as hardening may occur.
In Republished Japanese-Translation of PCT Application, Publication No. 2009-118783, since the transition metal layer alone generates heat by remote hydrogen plasma processing irrespective of device surface unevenness, transition metal layer patterns, etc., the transition metal layer alone can be uniformly heated. Nonetheless, in Republished Japanese-Translation of PCT Application, Publication No. 2009-118783, high-density plasma is not created because the pressure is lowered to increase the lifetime of the hydrogen atoms. Therefore, a problem arises in that the hydrogen atom density becomes low, whereby rapid heating is not possible. In practice, since plasma processing at a low electrical power of 200 W to 500 W is performed for a long period, during the plasma processing, constituent portions other than the transition metal layer (e.g., the entire device) are heated by a transfer of the heat generated by the transition metal layer and as a result, device properties may degrade.